Valorization of an underused sugar derived from hemicellulose: efficient synthesis of 5-hydroxymethylfurfural from mannose with aluminum salt catalyst in dimethyl sulfoxide/water mixed solvent

Converting saccharides into 5-hydroxymethylfurfural (5-HMF) has attracted more and more research interest under the background of global energy shortage. A lot of effort has been devoted to the conversion of fructose and glucose. However, other underused sugars receive very limited attention, although some of them have considerable reserves in nature as well. In this work, mannose, a major component from hemicellulose, was effectively converted into 5-HMF under mild conditions. AlCl3·6H2O exhibited superior activity among the tested catalysts, achieving a maximum 5-HMF yield of 60% in a dimethyl sulfoxide (DMSO)/water mixed solvent at 130 °C within 45 min. Adding an appropriate amount of water in DMSO could suppress some side reactions while preserving the reactivity of mannose dehydration. Mannose showed a comparable reactivity to fructose in the employed catalytic system. The studied system was also effective in the conversion of di/trisaccharides such as cellobiose and melezitose into 5-HMF. A number of control experiments with different catalysts and additives were conducted to elucidate the preliminary mechanism.